Free-piston Stirling machines have had control systems for ensuring useful power is generated by the machine while concurrently preventing overstroke of moving members that could lead to damage. One such control system uses valves or ports that detune the machine to change spring forces and/or generate damping. Such control systems are provided within the machine and can disrupt or unbalance the Stirling thermodynamic cycle which leads to inefficiencies. Such control systems are implemented internally. However, valves or ports on pistons or moving members tend to leak over time, tend to plug up from debris, and can fail over time. Furthermore, gas springs generally have high hysterisis loses. Additionally, valves do not generally perform well when subjected to abnormal or sudden load changes (i.e. transient loading conditions).
U.S. Pat. No. 4,642,547 to Redlich discloses one external electronic control system for preventing overstroke of moving members on a Stirling machine. Redlich teaches a control system that provides a fixed voltage at discrete power levels. However, such control system is inefficient, uses more components, and is more costly and complex.